Laser Lab - Basic Version

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Laser Lab – Basic
In this lab, you will explore the behavior of light. You will observe reflection and refraction of a laser beam in jello, and use a diffraction pattern to measure the width of a hair.

Key Concepts:
• Light always travels in a straight line except when it interacts with an object through reflection, refraction, or diffraction • Reflection: when a light wave hits an obstacle (eg: a mirror), it can reflect (bounce back). • Refraction: Some obstacles (eg: glass, water) allow light to pass through, but slow it down. In this case, the light ray bends as it crosses the object boundary. The angle of refraction depends on the speed of light in the object. • Diffraction: Light waves can bend around a small obstacle or when passing through a narrow slit. Light waves generated by points around the obstacle overlap to create patterns of light and dark bands.

Part 1: Refraction of light in Jello
You should have a sheet of paper labeled “Diagram 1” that shows your experimental setup. You will have a semi-circular block of jello sitting over the semicircle in the diagram. The labeled lines indicate the direction for shining your laser into the jello. The laser will be pointing into the center of the flat side of the jello. The straight dashed line down the center of the diagram is called the normal line . First, make some predictions: 1. When you shine the laser along the normal line (line A), what direction will the laser beam have inside the jello? Draw the path in the diagram to the right: 2. Do you think that light travels slower or faster in jello as compared to in air? 3. When you shine the laser along line C, will the light beam bend away from the normal line or towards the normal line? Draw what you think the approximate path will look like on the diagram to the right:

Now test what actually happens: 1. Take the semi-circular block of jello and slide it onto the experimental setup diagram over the shape labeled “jello”. Do NOT take the off its wax paper. Your jello might be a different size from the diagram but you must have the curved side of the jello facing away from you and the center of the flat side should match the black dot. 2. Line up the laser along line A and shine it towards the central dot. Your partner should use a pencil to mark the point where the light beam comes out the curved side of the jello. Make sure you make the mark on the actual diagram, not the wax paper. 3. Shine the laser along lines B, C, and D and mark where the beam comes out on the curved side in each case. You may want to label your marks A, B, C, D so you don't forget which is which. 4. Using a ruler, draw lines connecting each of the marks you made to the central dot. Make the lines extend past the points that you marked (further towards the outside of the diagram). 5. Use a protractor to measure the angle from the normal for the incoming laser beam and for the path the beam takes in the jello. Fill in the first 2 columns in the table below:

Angle from normal Incoming beam (θ air) A B C D 0° 30° Beam in jello (θ jello)

Did the laser beam bend towards or away from the normal line when entering the jello?

Was your original prediction correct (does light travel faster or slower in jello than in air)? Challenge question: Imagine a fish at the bottom of a pool of water. You are looking at it from above the water. Note that light travels slower in water than in air. (a) Draw a ray of light from the penny to your eye in the picture. Make sure the ray bends in the correct direction at the air-water interface due to refraction! (b) When your brain processes information from your eyes, it always assumes that the light you saw traveled in a straight line. Draw a (straight!) dashed line from your eye to where the fish appears to be. (c) Do objects under water appear to be shallower, deeper, or at the same depth as they really are?

Part 2: Total internal reflection and fiberoptics
1. Transfer your jello, still on its wax paper, onto diagram 2. Shine the laser into the curved side, so that the beam goes through the jello and hits the center of the flat side at an angle. Have another student hold a curved sheet of paper vertically as indicated on the diagram. 2.You should see a spot appear on the vertical sheet of paper after the laser leaves the jello. Using a pencil, make a mark on the diagram just below this dot. Mark down the line along which the laser goes into the jello. 3.Draw a line connecting your marks to the black dot at the center of the flat side of the jello. Did the laser beam bend away from the normal or towards the normal when it left the jello and entered the air? 4. Now change the angle of your incoming beam so that the laser passes through and hits the flat side of the jello at a greater and greater angle. When you shine the light at a large angle from the normal, how many laser beams do you see in the jello? Explain where the extra beam(s) come(s) from.

Can you make the dot on the paper made by the laser leaving the jello disappear? This phenomenon is called “total internal reflection”. At a big enough angle from the normal, all of the laser light will be reflected and none will pass out into the air. Fiber-optic cables use total internal reflection to transmit light and other signals over long distances. You can make a model for a fiberoptic cable out of jello. 6. Obtain a long strip of jello from your teacher. Shine the laser into one end of it at an angle, making the laser reflect off the sides of the strip. How many times can you make the laser beam bounce off the walls before leaving the jello? Draw a diagram of its path in the strip.

Although light normally travels in a straight line, with a fiber-optic cable, we can make it curve around a corner! Try curving the jello strip. Can you still make laser light come out of the opposite end of the jello from where you shine the laser?

Part 3: Diffraction- Measure the thickness of your hair!
1. One student should volunteer a piece of hair from their head. The hair should be placed tightly across the center of the laser aperture and a piece tape attached around the laser to hold the hair in place (please see diagram below). hair laser aperture laser scotch tape to fasten hair to sides of laser 2. Measure (in cm) the distance from the edge of a table to a blank wall. Distance from table to wall is L = ________________ cm 3. Rest the laser on the edge of the table to keep it steady. Shine the laser towards the wall. A diffraction pattern of bright and dark bands should be visible

x

L
4. Measure (in cm) the width of the central bright spot. That is, measure the distance between the centers of the dark bands on either side of the center. Distance between dark bands is x = ________________cm 5. Your laser should have written on it the wavelength (λ ) for the light that it produces. Wavelength of laser beam is λ = ________________ nm 6. According to the laws of diffraction, the width, w, of the hair (obstacle) will equal to:
w = l ´ L ¸ x ¸ 1000 = ______________ mm

7. The average thickness of human hair is 100μm. This thickness can range anywhere from 10 μm to 200 μm. Is the thickness of your hair within the common range?

Do you have thin, thick or about average hair? 8. Compare your results with other students. Does hair color or ethnicity tend to correspond to particularly thin or thick hairs? If, instead of human hair, you taped a hair from a horse’s tail across the laser pointer, would you expect the peaks to be closer together or further apart?

Optional: Test your prediction with a piece of horse-hair provided by your teacher. How many times thicker is the horse hair compared to your hair?

This page intentionally left blank for separating diagrams on two-sided copies.

Diagram 1

Diagram 2
normal line

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